Exercise can be divided into two metabolic categories: anaerobic and aerobic. Aerobic exercise is a "pay-as-you-go" process in which your body can meet its fuel needs using the oxygen you breathe in, and can be carried on hours without a break; examples are easy running, walking or gentle cycling. Anaerobic exercise, in contrast, is a "power-me-now-and-I'll-pay-you-later" scheme in which inspired oxygen is insufficient to meet your fuel demands, as in sprinting, hard cycling and rowing. Anaerobic exercise consists of numerous stages from planning to recovery.

Planning Your Workout

Before engaging in anaerobic exercise, you should have a specific plan for going about it. For example, if you're a sprinter, you need to do frequent anaerobic speed workouts, but you can't just dash around haphazardly until you're worn out; you should know the number of sprints you'll do in a session, how long they will last, how much rest to take between sprints, and how many times a week you can do such workouts while getting enough recovery between them to reap their benefits.

The Warm-Up

You should prepare your muscles for high-intensity exercise by doing five to 10 minutes of easy jogging and perhaps a dynamic stretching routine. This, incidentally, is a good idea before any sort of aerobic exercise as well; warmer muscles make stronger muscles.

The First Few Seconds: ATP from Phosphate

Your muscles can store a very limited amount of the substance ultimately responsible for muscle contraction -- ATP. When you explode into motion from rest, you use up this store within about two seconds. Then, your muscles are able to make more ATP from a similarly limited supply of creatine phosphate; this lasts for another four to six seconds. So by the time you're about 10 seconds into, say, an all-out sprint, you've depleted your so-called alactic energy reserves.

The Next Half-Minute: Glycolysis

As you continue motoring along, your muscles switch to using glycogen for energy. This compound, your body's storage of glucose, is hydrolyzed in muscle and used to create more ATP. This can sustain all-out work for about another 30 seconds, bringing you to roughly 45 seconds in all, the time it takes for a world-class male track runner to race a quarter-mile.

Hydrogen Ion and Lactate Production

The by-products of glycolysis include lactate and hydrogen ions, which outside the body usually combine to form lactic acid. Contrary to very pervasive belief, lactate does not cause the "burn" you feel in muscles; nor does it interfere with their work. It's the hydrogen ions that cause your gears to start grinding because they cause your intramuscular pH to drop -- that is, your muscles become acidic.

Fatigue and Exhaustion

As you exhaust your glycogen stores and hydrogen ions pile up in your muscles and bloodstream, a phenomenon called metabolic acidosis sets in. At this point, no matter how determined you are, any efforts to continue maintaining your rate of work are futile, and your performance drops off dramatically. Meanwhile, your body attempts to offset the acidosis by "buffering" the hydrogen with bicarbonate and the amino acid histidine.

The Cooldown

Now that you've finished your workout, it's time to revisit easy jogging. Doing five to 10 minutes of this helps clear the metabolic by-products of anaerobic metabolism from your system, possibly leaving you less sore the next day and hence in a much better position to resume regular training.

Review and Recovery

Just as important as doing the hard work an anaerobic session entails is evaluating how well it met your expectations -- for example, whether your plan was within your capabilities without being too easy and the extent to which the workout makes your competitive goals appear realistic. Also remember that as a rule, you need two full days of easy workouts between anaerobic ones.

References

About the Author

L.T. Davidson has been a professional writer and editor since 1994. He has been published in "Triathlete," "Men's Fitness" and "Competitor." A former elite cyclist with a Master of Science in exercise physiology from the University of Miami, Davidson is now in the broadcast news business.